# std::is_base_of

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< cpp‎ | types

Defined in header `<type_traits>` |  |
template< class Base, class Derived > struct is_base_of; |  | (since C++11)

`std::is_base_of` is a BinaryTypeTrait.

If `Derived` is derived from `Base` or if both are the same non-union class (in both cases ignoring cv-qualification), provides the member constant value equal to true. Otherwise value is false.

If both `Base` and `Derived` are non-union class types, and they are not the same type (ignoring cv-qualification), `Derived` should be a complete type; otherwise the behavior is undefined.

If the program adds specializations for `std::is_base_of` or `std::is_base_of_v`(since C++17), the behavior is undefined.

### Helper variable template

template< class Base, class Derived > constexpr bool is_base_of_v = is_base_of<Base, Derived>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `Derived` is derived from `Base` or if both are the same non-union class (in both cases ignoring cv-qualification), false otherwise (public static member constant)

### Member functions

operator bool | converts the object to bool, returns value (public member function)
operator() (C++14) | returns value (public member function)

### Member types

Type | Definition
`value_type` | bool
`type` | std::integral_constant<bool, value>

### Notes

std::is_base_of<A, B>::value is true even if `A` is a private, protected, or ambiguous base class of `B`. In many situations, std::is_convertible<B*, A*> is the more appropriate test.

Although no class is its own base, std::is_base_of<T, T>::value is true because the intent of the trait is to model the "is-a" relationship, and `T` is a `T`. Despite that, std::is_base_of<int, int>::value is false because only classes participate in the relationship that this trait models.

### Possible Implementation

    namespace details
    {
        template<typename B>
        std::true_type test_ptr_conv(const volatile B*);
        template<typename>
        std::false_type test_ptr_conv(const volatile void*);

        template<typename B, typename D>
        auto test_is_base_of(int) -> decltype(test_ptr_conv<B>(static_cast<D*>(nullptr)));
        template<typename, typename>
        auto test_is_base_of(...) -> std::true_type; // private or ambiguous base
    }

    template<typename Base, typename Derived>
    struct is_base_of :
        std::integral_constant<
            bool,
            std::is_class<Base>::value &&
            std::is_class<Derived>::value &&
            decltype(details::test_is_base_of<Base, Derived>(0))::value
        > {};

### Example

Run this code

    #include <type_traits>

    class A {};
    class B : A {};
    class C : B {};
    class D {};
    union E {};
    using I = int;

    static_assert
    (
        std::is_base_of_v<A, A> == true &&
        std::is_base_of_v<A, B> == true &&
        std::is_base_of_v<A, C> == true &&
        std::is_base_of_v<A, D> != true &&
        std::is_base_of_v<B, A> != true &&
        std::is_base_of_v<E, E> != true &&
        std::is_base_of_v<I, I> != true
    );

    int main() {}

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2015 | C++11 | the behavior might be undefined if `Derived` is an incomplete union type | the base characteristic is std::false_type in this case

### See also

is_virtual_base_of (C++26) | checks if a type is a virtual base of the other type (class template)
is_convertibleis_nothrow_convertible (C++11)(C++20) | checks if a type can be converted to the other type (class template)
derived_from (C++20) | specifies that a type is derived from another type (concept)

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